Network Bridging Spanning Tree Algorithm

Network bridges exchange information on network topology with the help of a spanning tree algorithm (STA). This algorithm is designed to prevent forwarding topology loops in networks with redundantly bridged segments. In a network with topology loops, bridges can cause forwarding storms by relaying the same frame endlessly from one segment to another.

The STA allows a bridge to select the most efficient path when faced with multiple paths in a multisegment network. It provides each bridge with the information required to disable one or more of its ports to ensure that the network topology is loop-free. The STA can easily adapt to topology changes. It uses timers to ensure that the forwarding topology is recomputed if a bridge is removed or shut down.

In a multisegmented network, each bridge must have a globally unique identification number. Most implementations take advantage of the global uniqueness of Ethernet MAC addresses and use the lowest-numbered MAC address from among its adaptors as the bridge identification number.

Network bridges communicate with each other through special packets, which can be referred to as STA packets. STA packets submit information about the network forwarding topology from one bridge to another. Each STA packet contains the following pieces of information.

• The identifier of the bridge that the transmitting bridge assumes to be the root bridge.
• The cost (number of hops) from the transmitting bridge to the bridge it believes to be the root bridge.
• The identifier of the transmitting bridge.

See Also

Network Bridging | Port States | Spanning Tree Algorithm Functionality

 Last updated on Friday, April 09, 2004

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